The field of the invention is ultrasonic transducers for use in flow meters.
This invention is concerned with transducers for use in ultrasonic flow meters. The ultrasonic flow meter is an instrument that uses ultrasonic waves to measure the flow rate of a fluid. In operation, two transducers are placed on opposite inside wall surfaces of a pipe through which the fluid is flowing. Acoustic waves of ultrasonic frequency are generated by a first transducer and travel through the fluid to reach the second transducer. The time taken by the acoustic wave to travel from one transducer to the other, the transit time, is a function of the flow velocity of the fluid. Thus, by measuring the transit time, one can determine the fluid velocity and other relevant flow parameters.
Many transducer structures have been developed to generate and detect acoustic waves in a fluid.
One of the problems of prior transducers is that they are generally large in size and protrude a considerable distance inward from the pipe wall and into the path of the flowing fluid. This poses a problem, particularly if the diameter of the pipe is small. There is a certain minimum pipe diameter below which the present transducers cannot be used. A second problem is that the protrusion into the flow stream can allow rags, debris, and other large particulate matter to foul the transducer. This is not a problem in large pipes where the transducer does not significantly project into the flow stream, but this can be a problem in pipes of small diameter. A third problem with the current transducers is the effect they have on the flow streams themselves. The sensor can disturb the flow stream sufficiently to create erroneous flow data.
Lynnworth, U.S. Pat. No. 4,735,097, illustrates non-invasive type ultrasonic transducers which are supported at an angle in housings attached to the outside wall of a pipe to generate Rayleigh-like surface waves. These transducers are not mounted in the flow stream and the signals generated in this system are attenuated by the pipe wall.
Magori, U.S. Pat. No. 4,375,767, discloses a planar transducer using an interdigital array in which the periodicity of the interdigital array must satisfy a condition d=xcex/cos xcex1, where d=is the periodicity of the interdigital structure, xcex is the wavelength of acoustic waves in the flowing medium and a is the angle of radiation of acoustic waves into the flowing medium, which is dependent on periodicity, d.
The invention provides a compact, planar transducer configuration having a transducer which is not limited by the periodicity condition of the prior art. Such a transducer, which should essentially be in the form of a thin, planar device can be mounted flush with the pipe wall resulting in minimal protrusion into the flow channel. The transducer of the present invention utilizes surface acoustic waves (SAW) or plate acoustic waves (Lamb waves) to couple energy into or out of bulk acoustic waves propagating in the fluid.
The transducer of the present invention cannot only be fabricated in a small size, but it is also much more efficient than existing transducers. This provides an output signal in response to a much lower voltage applied to the input transducer. This is particularly important in applications where engineering considerations set an upper limit to the maximum driving voltage that can be applied across the input transducer. The lowering of the drive voltage can result in significant cost savings in the packaging of the device.
The invention also provides advantageous configurations for packaging the transducer, so that it is sealed from contact with the fluid being measured while providing electrical connections to an instrument outside the pipe.